1. Field of the Invention
The present invention relates to a thin-film forming apparatus, a thin-film forming method, a piezoelectric-element forming method, a droplet discharging head, and an ink-jet recording apparatus.
2. Description of the Related Art
Conventionally, a droplet discharging head of an ink-jet recording apparatus used as an image recording apparatus or image forming apparatus, such as a printer, a facsimile machine, and a copier, includes a nozzle for discharging an ink drop, a pressurizing chamber (also referred to as an ink flow path, a pressurizing liquid chamber, a pressure chamber, a discharge chamber, or a liquid chamber, and the like.) into which the nozzle leads, and a structure to pressurize ink in the pressurizing chamber, and discharges an ink drop from the nozzle by pressurizing the ink. Such structures to pressurize ink include an electro-mechanical transducer (hereinafter, referred to as a “piezoelectric element”), a thermoelectric transducer such as a heater, an energy generating means composed of a diaphragm forming a wall surface of the ink flow path and an electrode opposed to the diaphragm, and the like.
An example of a configuration of a droplet discharging head using a piezoelectric element is described. FIG. 16 is a cross-sectional view illustrating an example of a configuration of a droplet discharging head 400. As illustrated in FIG. 16, in the droplet discharging head 400, a pressure chamber 421 is formed by a nozzle plate 410, a pressure chamber substrate 420 (a silicon (Si) substrate), and a diaphragm 430. A nozzle 411 leading into the pressure chamber 421 is provided on the nozzle plate 410, and a piezoelectric element 440 is provided on the diaphragm 430 via an adhesion layer 441. The droplet discharging head 400 is configured to discharge an ink drop from the nozzle 411 of the nozzle plate 410 by causing the piezoelectric element 440 to vibrate the diaphragm 430 thereby pressurizing the pressure chamber 421. An ink-jet recording apparatus forms an image on a recording medium such as a sheet using a recording head in which the droplet discharging heads 400 corresponding to pixels are aligned at predetermined intervals.
The piezoelectric element 440, which is a main part of the droplet discharging head 400, is formed, by means of thin-film formation, by depositing a lower electrode 442, an electro-mechanical transducer film 443, and an upper electrode 444 in this order on the adhesion layer 441. The lower electrode 442 and the upper electrode 444 are electrodes to make electrical input to the electro-mechanical transducer film 443. The electro-mechanical transducer film 443 transduces electrical input made by the lower electrode 442 and the upper electrode 444 into mechanical deformation. Specifically, lead zirconate titanate (PZT) ceramics and the like are used in the electro-mechanical transducer film 443, and these consist primarily of a plurality of metal oxides, which is generally referred to as a metal composite oxide.
A conventional method to form the piezoelectric element 440 is as follows. First, the electro-mechanical transducer film 443 is deposited on the lower electrode 442 by a well-known film formation technique, such as various vacuum film forming methods (for example, a sputtering method, a metalorganic chemical vapor deposition (MO-CVD) method using a metal organic compound), a vacuum deposition method, and an ion plating method), a sol-gel method, a hydrothermal synthesis method, an aerosol deposition (AD) method, or a metal organic decomposition (MOD) method, and the upper electrode 444 is formed on the electro-mechanical transducer film 443, and thereafter, patterning of the upper electrode 444 is performed by means of photolithography etching, and patterning of the electro-mechanical transducer film 443 and the lower electrode 442 is performed in the same manner, thereby individualizing them.
The metal composite oxide, especially PZT, is not an easy processing material for dry etching. An Si semiconductor device can be easily etched by means of reactive ion etching (RIE); however, this kind of material increases plasma energy of ion species, so special RIE with a combination of ICP plasma, ECR plasma, and helicon plasma is performed (this causes high production costs of production equipments). Furthermore, a PTZ cannot acquire a high selection ratio of etch rates to a base electrode film. In particular, non-uniformity of the etching rate is fatal to a substrate with a large area. If a hard-to-etch PZT film is arranged on a desired region only prior to etching, the above-described manufacturing process can be omitted; however, such an attempt is unsuccessful with a few exceptions. Methods of producing a PZT film individually are the hydrothermal synthesis, the vacuum deposition method, the AD method, and an ink-jet method.
Hydrothermal synthesis: PZT is selectively grown on titanium (Ti) metal. By patterning a Ti electrode before growth, a PZT film is grown only on the patterned region. To obtain a PZT film having sufficient pressure resistance by this method, a relatively-thick film with the film thickness of 5 micrometers or more is preferable (if the film thickness is less than 5 micrometers, a dielectric breakdown easily occurs in a thin film in applying an electric field, so that a thin film having a desired property cannot be obtained). Furthermore, in a case when electronic devices are formed on a Si substrate, hydrothermal synthesis is performed in strong alkaline aqueous solution, so that the protection of the Si substrate against etching becomes necessary.
Vacuum deposition: A shadow mask is used in producing an organic electroluminescent (EL) device, and patterning is performed on a luminous layer; a PZT film is formed at a substrate temperature of 500 to 600 degrees in centigrade. This is because a composite oxide has to be crystallized to emerge the piezoelectricity property, and the above range of the substrate temperature is required to obtain a crystalline film. A shadow mask is generally made of stainless steel, and the feasibility of a disposal shadow mask, which is incapable of sufficient masking due to a difference in the coefficients of thermal expansion between the Si substrate and the stainless material, is low. In particular, it is less appropriate to use a shadow mask made of stainless steel in the MO-CVD or the sputtering method in which shadow-less deposition due to gas scattering is extensive in deposited film growth.
AD method: There is known a method to form a resist pattern by photolithography prior to etching and form a PZT film on an area without resist treatment. The AD method is, similarly to the hydrothermal synthesis method described above, suitable for thick film growth and not for a thin film with the thickness of 5 micrometers or less. Furthermore, because the PZT film is deposited on a resist film, a liftoff process is performed after the deposited film partly is removed by a polishing process. The process of uniformly polishing a large area is cumbersome and complicated, and the resist film has no resistance to high temperature. Therefore, AD film is formed at room temperature, and the film is converted into a piezoelectric film by a post-annealing process.
Ink-jet method: As a prior art related to the ink-jet method in which a metal wiring pattern is formed by droplet discharging followed by drying and baking by laser light irradiation, patent documents (Japanese Patent No. 4353145, Japanese Patent No. 4232753, Japanese Patent Application Laid-open No. 2007-152250, Japanese Patent Application Laid-open No. 2007-105661) and non-patent documents (K. D. Budd, S. K. Dey, D. A. Payne, Proc. Brit. Ceram. Soc. 36,107 (1985) and A. Kumar and G. M. Whitesides, Appl. Phys. Lett. 63, 2002 (1993)) are known. Japanese Patent No. 4353145 discloses a thin-film forming apparatus having an ink-jet mechanism and a laser irradiation mechanism, and the thin-film forming apparatus includes a drawing system for discharging a droplet to a target in a workspace and is capable of performing quick and accurate positioning of a laser spot to the discharged droplet. Japanese Patent No. 4232753 discloses a technology for efficient drying/baking of a discharged droplet containing a functional material by irradiating the droplet with a laser light accurately. Japanese Patent Application Laid-open No. 2007-152250 discloses a technology to suck an evaporated component from a droplet at a suction rate corresponding to the fluidity of the droplet thereby improving the controllability of the pattern formation. Japanese Patent Application Laid-open No. 2007-105661 discloses a technology in which an open-close mechanism is provided at an irradiation port of laser light so as to close the irradiation port when no laser beam is irradiated, thereby maintaining the laser optical characteristics. K. D. Budd, S. K. Dey, D. A. Payne, Proc. Brit. Ceram. Soc. 36,107 (1985) presents a technology related to formation of a thin film made of metal composite oxide by the sol-gel method. A. Kumar and G. M. Whitesides, Appl. Phys. Lett. 63, 2002 (1993) states that alkanethiol can be formed on an Au film as a self-assembled monolayer (SAM), and a SAM pattern is transferred by a micro-contact printing method using this phenomenon to be used in a subsequent process such as etching.
However, in the conventional ink-jet method described above, a thin film is formed by directly irradiating the surface of a droplet with a laser light thereby drying/baking the droplet to result in characteristic degradation due to drying of the droplet from the surface. In an ink-jet recording apparatus using a piezoelectric element having such characteristic degradation, the quality of an image is reduced.